Problem: Which of these numbers is prime? ${11,\ 22,\ 33,\ 85,\ 91}$
A prime number has exactly two factors: 1 and itself. The factors of 11 are 1 and 11. The factors of 22 are 1, 2, 11, and 22. The factors of 33 are 1, 3, 11, and 33. The factors of 85 are 1, 5, 17, and 85. The factors of 91 are 1, 7, 13, and 91. Thus, 11 is a prime number.